CAUSTIC GOUGING IN BED TUBES OF HIGH PRESSURE AFBC …

CAUSTIC GOUGING IN BED TUBES OF CAUSTIC GOUGING IN BED TUBES OF

HIGH PRESSURE AFBC BOILERSHIGH PRESSURE AFBC BOILERS

BY K.K.PARTHIBAN,BY K.K.PARTHIBAN,ENERGY CONSULTANTENERGY CONSULTANT

Relative Corrosion of Carbon Steel by HCl and NaOH at 310oC.

Minimum Corrosion rate at ~pH25 = 11

Introduction Introduction

Boiler water has to be at a pH of 9 to 10.2 in Boiler water has to be at a pH of 9 to 10.2 in order to preserve the magnetite layer of steel.order to preserve the magnetite layer of steel.This is done by dosing Tri This is done by dosing Tri --sodium phosphate sodium phosphate or by a mix of trior by a mix of tri--sodium & sodium & didi--sodium sodium phosphates in boiler water through HP dosing phosphates in boiler water through HP dosing system. system.

Introduction Introduction

In case there is a mechanism of concentration of the alkalinity of boiler water anywhere in the evaporative circuit, caustic gouging / caustic attack occur. This is what happened in the some of the high pressure AFBC installations that came up in the last two years.

What is caustic gouging? What is caustic gouging?

Caustic gouging of metal is the loss of metal due to a concentration of alkalinity There is no hardness change in the tube. Simply the metal is removed in evaporative circuits.

A recall of the past A recall of the past

In the year 1990 in 30 TPH AFBC boiler. The bed evaporator tubes failed in 4th compartment within six months after the boiler was commissioned. There was gross deviations in water chemistry. The alkalinity level was very high and there was clear indication of free OH presence in boiler water. There was a mechanism of concentration of alkalinity.

A recall of the past A recall of the past –– failure mechanismfailure mechanism

The load on the boiler was less, the 4th compartment was always kept under slumped condition. Circulation is a function of heat flux in the evaporator tube. When the heat input is less, the steam generation is less. The velocity of the water in such tubes is expected to be too low, resulting in very slow movement of water.

A recall of the past A recall of the past –– failure mechanismfailure mechanism

Circuits with low velocity and some steaming would lead to concentration of boiler water. The local water alkalinity could reach very high level thus resulting in break down of magnetite layer. The failed tubes had a groove mark as shown in photo 1. The leakage developed through a pin hole in the groove portion where the material is continuously removed by the caustic action.

A recall of the past A recall of the past

CORROSION FAILURE DUE TO CAUSTIC

TOP OF TUBE IDLE COMPARTMENT

Caustic gouging is related to loss of metal Caustic gouging is related to loss of metal by caustic concentration mechanisms or by by caustic concentration mechanisms or by

gross deviation in water chemistry. One gross deviation in water chemistry. One such mechanism is stratification of steam / such mechanism is stratification of steam /

water inside sloped tubeswater inside sloped tubes. .

Boiling & two phase flow inside tubesBoiling & two phase flow inside tubes

Types of flow pattern of water / steam mixture inside tubes of evaporative circuits

Flow patterns Flow patterns

Steam and water have different densities at different pressures. At the lower circulation ratios, the steam and water can exist in several flow patterns inside the tubes. Flow pattern means distribution of the steam with respect to water.

Parameters that decide flow patterns Parameters that decide flow patterns

Density difference between the steam / water. Surface tension of water to adhere to tube wall.Steam / water ratio.Gravity – which tends to pull the water to bottom of pipe and push the steam to the top of pipe – specifically in inclined tube.

Flow patterns in vertical tubesFlow patterns in vertical tubes

Flow patterns in Flow patterns in vertical tubesvertical tubes

As the quality, that is steam to water ratio increases, the flow patterns change from bubbly to wispy annular flow. The mass flux would decide the flow pattern inside tube. The flow has to be bubbly flow so that the water / steam flow well mixed inside tube without any steam blanketing inside the tube. Otherwise the metal temperature would go up resulting in tube bursts.

The phases tend to separate due to density difference causing a form of stratified flow to be very common.

Flow patterns in horizontally oriented tubesFlow patterns in horizontally oriented tubes

Flow patterns vary from that of vertical tubes due to gravitational forces which act perpendicular to the direction of flow. As the steam fraction increases, the flow pattern changes from Plug to annular flow.

Flow patterns in horizontally oriented tubesFlow patterns in horizontally oriented tubes

The above figures is the well known representation of the steam bubble behavior inside a horizontally oriented tube. It is the designer’s job to design the evaporation circuit with proper downcomer and riser arrangements, evaporator tube arrangements to ensure the bubbly flow prevails in each part of the boiling circuit.

In circuits of less heat input, the circulation will be low.If steaming is possible in such a circuit, the steam bubble may rise to the top and form bubbles adhering to tube wall at 12o clock position.As the steam bubble sticks to the top, the flow is further retarded. As the bubble grows in size it leaves the place.The water begins to concentrate as the steam alone leaves the place..

What can happen inside the sloped tubes What can happen inside the sloped tubes under very less heat input?under very less heat input?

The tubes may not fail due to metal temperature excursion because the gas side temperature may be low. But corrosion mechanism begins and results in a gouged tube as seen above.

What can happen inside the sloped tubes What can happen inside the sloped tubes under very less heat input?under very less heat input?

High heat input would help in forming more steam bubbles & facilitate the growth of the bubble. As the bubble size increases, it would raise to the top of the tube in the absence of sufficient horizontal velocity or when the tube is horizontal or inclined to low angle. Now the steam forms a blanket inside the tube, may be for a moment, before being washed away by water. Part of the trapped water between bubbles can form steam leaving a chemically concentrated water behind.

What can happen inside bed tube under What can happen inside bed tube under situation of high heat input, horizontal / low situation of high heat input, horizontal / low

sloped orientation / less velocity?sloped orientation / less velocity?

When the blanket is heavy and in the absence of horizontal velocity component, the steam would get superheated and tube would have failed due to high metal temperature. The failure would have been instantaneous. This is called dry out and the heat flux is called critical heat flux.

What happens when the steam blankets the What happens when the steam blankets the tube? tube?

Long term overheating is also due to retarded flow- but at lower operating pressure.Short term over heating due to steam blanketing occurs at higher pressures

Long term overheating / Sudden over heating.Long term overheating / Sudden over heating.

When there is a horizontal velocity component the steam bubbles and water wetting alternately take place at the top of the tube. This is described as intermittent flow. Now we have a mechanism of local concentration of solids due to evaporative boiling. The water going along the top of the tubes has a different velocity as compared to the stream of water going along bottom of tubes.

A closer look at gouging mechanism A closer look at gouging mechanism

This boiler water sample represents an average of all the water within the boiler.As such it contains water subjected to both the most severe and the least severe conditions within the boiler. Water chemistry except for presence of corroded iron at drum internals, does not indicate concentration of solids / chemicals inside the low sloped bed tubes.


The low slope tubes may prevent adequate mixing with the bulk liquid phase and over time, a concentration gradient will develop between the bulk boiler water and the localized solution in contact with the magnetite surface. Localized concentrations for some species may be two or more orders of magnitude higher than in the bulk boiler water (Cohen et al., 1962).


In gouging zones, NaOH and NaCl, with relatively high solubility tend to stay in solution.Salts with limited solubility, such as those incorporating phosphate species, may undergo precipitation. Wherever the tubes had failed due to gouging, the analysis indicated phosphate salts..


In one case, the failures were seen at 300mm from the waterwall. In another case, the gouging was seen in the upper portion of hairpin bed coil. In another case, the failures were frequent above under bed fuel nozzles. Leak occurred below stud when studs were provided all around.

Locations of failures Locations of failures

When there were two coils in one plane the failure was at the outer coil. Depending upon the water outlet temperature at economiser, the water inside the bed tubes begin to boil at a distance from the wall. The gouging mark was seen inside the tube after the steaming point.

Locations of failures Locations of failures

The gouging type failures have been seen in at least six installations where the slope of the bed tube happened to be less than 5 deg. The steam to water fraction happened to be higher and the overall circulation ratio was estimated close to 8. Mass velocities in the bed tubes could be lower than what was estimated.

Causes of failures Causes of failures

When it happened, the immediate concern was to how to keep the boiler on line before any remedial measures could be taken. Operating at lower loads / lower pressure will delay the failure. This is mainly due to the reason that the steam to water ratio is more at rated load & rated pressure. Operation of the boiler within narrow range of pH - phosphate Chemistry delayed the failure.

What needs to be done to delay the failure?What needs to be done to delay the failure?

Rifle bore / internally ribbed tubes have been used to enhance mixing of steam and water. In the installations where the tubes had already failed this was the only solution.

What was done to prevent the future failure?What was done to prevent the future failure?

Rifle bore tubes have spiral ribs which impart swirling motion to steam-water mixture flowing through the tubes. The mixing of steam & water ensures the chemistry of water inside the tube is the same as that of bulk water.

What was done to prevent the future failure?What was done to prevent the future failure?

Irrespective of design of Boiler, the boiler water chemistry should be free of caustic.In any boiler there are locations where the circulation may be less thus offering sites for concentration of Hydroxide alkalinity.

Parameters which cause gougingParameters which cause gouging-- water chemistrywater chemistry

5-1015-2515 -2520 -4020 -40Phosphate residual, ppm

9.4 to 9.79.8 to 10.29.8 – 10.210 – 10.510 – 10.5PH at 25 deg C

101 & Above61 to 10041- 6021 -40Up to 20 Drum operating pressure bar (g)

Parameters which cause gougingParameters which cause gouging-- water chemistrywater chemistry

A narrow band of pH – phosphate regime is maintained by adopting a mix of tri-sodium phosphate & di-sodium phosphate. This is a single main factor for caustic gouging effect.

The factor for stratification is the included bed coil angle. Higher the angle there is less risk of stratification of steam &water. Thus gouging can be affected.

Parameters which lead to stratification of steam Parameters which lead to stratification of steam ––Bed coil inclination angle

Higher the volatile matter in coal, the coal injected in to the bed would tend to burn as a flame in the bed at fuel feed points.Thermocouples are located usually away from the fuel feed point. Thus the local flame temperatures could be plus 150 deg C. This seen by the melting of coal ash in some installation. The heat flux is now localized and thus steam/ water ratio can be more above the fuel feed points. The flow can become intermittent.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––fuel characteristics

Presence of large amount of fines in coal can lead to localized combustion. Heat input can be more due to this fact.When bed material is locally fluidised due to presence of iron / heavier particles, the flame shape up differently near the coal nozzle and thus heat input is localized. The bed will be a spouting bed.At low circulation ratios, this can cause steam stratification.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––fuel characteristics

Design defect such as undistributed downcomer can cause gouging effect. The tube away from downcomer gets fed less and thus steam fraction in water increases causes stratification phenomenon.This is critical for higher pressures.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––downcomer & riser arrangements

As the operating pressures go up CR comes down.Depending on steam generation in panels / boiler bank / bed coils, the feeding circuits / riser arrangements have to be separate.This is critical for higher pressures.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––downcomer & riser arrangements

Inadequate bed heat transfer area will lead to high bed temperature when boiler goes on line.The heat flux on bed tubes goes high.Managing with more studs would increase the heat flux.In underbed feeding local heat flux at fuel feed point would be high.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––High heat flux -net

Flow can reverse / slow down in tubes if the compartments are shut for load turndown.As the heat input is cut, the circulation comes down leading to mechanism of caustic concentration.

Parameters which lead to steam blanketing Parameters which lead to steam blanketing ––operational mistakes

Case studies Case studies

4Bed coil slope, deg160Heat flux, kw/m2

0.94Min inlet velocity, m/s

11.4Min CR in circuit

The inlet velocity is low. Further the angle seemed to be the cause for failure.Unit operating with rifle tube now with angle increased to 5.5 deg.

650Min mass velocity, kg/m2/s

Case study 1Case study 1–– 76 TPH, 88 kg/cm276 TPH, 88 kg/cm2

4Bed coil slope, deg

160Heat flux, kw/m2



The inlet velocity is too low.Further the angle seemed to be the cause for failure. Circulation arrangements were modified to overcome low mass velocity.Unit operating with rifle tube now.


Case study 2 Case study 2 –– 96 TPH, 88 kg/cm296 TPH, 88 kg/cm2


160Heat flux, kw/m2



The boiler tubes failed due to poor boiler water management.The unit is running with corrected chemistry. Yet the drum internal arrangement was not good. The drum surface indicated corrosion.


Case study 3Case study 3–– 95 TPH, 88 kg/cm295 TPH, 88 kg/cm2

Bad drum internal – bad water chemistry- had wide pH fluctuation

Bad drum internal Bad drum internal –– corrected water chemistrycorrected water chemistry

A good drum internal. Easy to maintain boiler water chemistry


230Heat flux, kw/m2



The heat flux, angle and possible localized heat flux seemed to be the cause. Rifle tubes were added. Mass velocity has come down by using higher diameter tubes. The heat flux is brought down due to increase in HTA. Boiler under observation.


Case study Case study –– 117 TPH, 105 kg/cm2117 TPH, 105 kg/cm2


230Heat flux, kw/m2



The failure took place when the bed temperatures were to be increased. Localized heat flux above fuel feed points & low angle are contributors for failure.


Case study Case study –– 127 TPH, 88 kg/cm2127 TPH, 88 kg/cm2

Fe2+ Ferrous ionFe3+ Ferric IonFeO Ferrous oxide – Iron II oxideFe2O3 Ferric oxide – Iron III oxideFe3O4 Ferrous ferric oxide – Iron II, III oxideFeOH2 Iron II hydroxideFeO(OH) Iron II oxide hydroxideFe(OH)3 Iron III hydroxide

The control of corrosion in boiler environment is based on maintaining conditions which enhance passive film formation. Magnetite, Fe3O4 is the preferred high temperature iron oxide form. Well crystallized (unhydrated) magnetite forms a dense layer resulting in excellent passivation.

Formation of magnetite layerFormation of magnetite layer

(Reaction -1) Fe + 2H2O <-> Fe +2 + 2OH- + H2(Reaction -2) Fe+2 + 2OH- <-> Fe (OH)2(Reaction -3) 3Fe(OH)2 <-> Fe3O4 + 2H2O + H2

Fe2+ Ferrous ionFe3+ Ferric IonFeO Ferrous oxide – Iron II oxideFe2O3 Ferric oxide – Iron III oxideFe3O4 Ferrous ferric oxide –Iron II, III oxideFeOH2 Iron II hydroxideFeO(OH) Iron II oxide hydroxideFe(OH)3 Iron III hydroxide

It is seen from reaction 2, that the hydroxyl ions are to help in continuous formation of Fe(OH)2 – that is soluble iron II oxide. Fe3O4 forms over the inner surface of tube offering a continuous protection. This would happen during alkali boil out stage. Passivation process is corrosion of active surface of Fe to a relatively inactive state of Fe3O4.

Formation of magnetite layerFormation of magnetite layer


Fe2+ Ferrous ionFe3+ Ferric IonFeO Ferrous oxide – Iron II oxideFe2O3 Ferric oxide – Iron III oxideFe3O4 Ferrous ferric oxide – Iron II, III oxideFeOH2 Iron II hydroxideFeO(OH) Iron II oxide hydroxideFe(OH)3 Iron III hydroxide

Under good operating condition, further oxidation of iron to magnetite at the metal surface is slow because the magnetite forms a fine, tight adherent layer. The film generally displays good adhesive strength in part because thermal coefficients of liner expansion for magnetite and layer are very similar. Therefore varying heat load and surface temperature do not cause undue stress between the film and the underlying metal surface.

Sustenance of magnetite layerSustenance of magnetite layer

(Reaction -4) - Fe3O4 + 2H2O <-> Fe(OH)2 + 2FeO.OH

Magnetite is considered to be a co polymer of the iron (II) hydroxide –FeO and iron (III) oxide –Fe2O3. In this system the bonds of divalent iron are more easily hydrolyzed. That is FeO can become FeO.OH by the above equation. The iron (II) hydroxide system (aqueous) is considered to be in solution equilibrium with magnetite as follows.

Fe2+ Ferrous ionFe3+ Ferric IonFeO Ferrous oxide – Iron II oxideFe2O3 Ferric oxide – Iron III oxideFe3O4 Ferrous ferric oxide –Iron II, III oxideFeOH2 Iron II hydroxideFeO(OH) Iron II oxide hydroxideFe(OH)3 Iron III hydroxide

Sustenance of magnetite layerSustenance of magnetite layer


(Reaction -5) - Fe(OH)2 + OH- <-> Fe(OH)3 <-> HFeO2 + 2H2O(Reaction -6) - Fe(OH)2 + 2OH- <-> Fe(OH)4 <-> FeO2

-2 + 2H2O

The solubility of magnetite layer in highly alkaline atmosphere (OH-) is explained as below. Under highly alkaline conditions the ferrous hydroxide in solution may react as follows.

Deterioration of magnetite layer in highly alkaline environment

Caustic attack occurs in this manner through activation of the carbon steel surface by removal of passive oxide layer inhibiting its formation. As a result the magnetite layer is dissolved in the form soluble ferrate ions as the equilibrium in reaction 4 is driven to the right via removal of ferrous oxide. The distortion of equilibrium illustrated in reactions 2 and 4 following the reactions 5 and 6 requires the presence of very high concentration of hydroxyl ions.


(Reaction -5) - Fe(OH)2 + OH- <-> Fe(OH)3 <-> HFeO2 + 2H2O(Reaction -6) - Fe(OH)2 + 2OH- <-> Fe(OH)4 <-> FeO2

-2 + 2H2O


Deterioration of magnetite layer in highly alkaline environment

Boiler blow down is based on permissible silica at higher steam pressures.But maintain it by CBD. When IBD is used, the PO4 drains out upsetting boiler water regime. To restore PO4 / pH it may take hours. It is virtually impossible to maintain required pH-PO4 chemistry in boiler water when shift operators change.

Caution on blow down practice

Caustic gouging can be prevented by proper boiler water chemistry.Stratified flow in inclined tubes leads to caustic gouging. Stratification defect can be overcome by rifle tubes.Stratification can be prevented by adopting higher angle of slopes at bed tubes. Localized high heat flux can cause stratification due to operational mistakes.

CONCLUSION

CAUSTIC GOUGING IN BED TUBES OF HIGH PRESSURE AFBC …

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